The present invention relates to an image forming apparatus in which an electrostatic latent image is formed on an image carrier. A developing roller is retractably brought into contact with the image carrier to develop the latent image as a visible toner image. The toner image is then transferred onto an intermediate transfer member. A first cleaner is always brought into contact with the image carrier to remove toner remaining thereon after the transfer is performed. A second cleaner is retractably brought into contact with the intermediate transfer member to selectably remove toner remaining thereon.
An image forming apparatus includes, around the periphery of a photosensitive drum serving as a rotary image carrier, a charger for charging the photosensitive drum uniformly along the rotational direction thereof, an exposer for forming an electrostatic latent image on the photosensitive drum, a developer for developing the electrostatic latent image, a transferer for transferring a toner image on the photosensitive drum onto a transfer member, a cleaner for cleaning the surface of the post-transfer photosensitive drum, etc. An electrostatic latent image formed on the photosensitive drum by the exposer is developed by the developer to form a toner image. The toner image thus formed is transferred onto a transfer member and residual toner on the post-transfer photosensitive drum is removed from the photosensitive drum by the cleaner.
In a case of a rotary-type full-color image forming apparatus that forms a full-color image, color toner images, formed on the photosensitive drum by the respective developing devices for yellow Y, magenta M, cyan C, and black K, are sequentially transferred onto an intermediate transfer member to superpose colors. For this reason, the developing cartridges of respective colors of toner are detachably mounted to a rotary development unit, and each time an electrostatic latent image of one color is formed on the photosensitive drum, the rotary development unit is driven by a color switching operation to move the developing cartridge of the corresponding toner by rotation to a developing position for a developing operation to be performed. Such a configuration is disclosed in Japanese Patent Publication Nos., 2002-82532A, and 2003-66801A, for example.
In the image forming apparatus, the electrostatic latent image on the photosensitive drum is formed by repetitively exposing a light beam modulated by an image forming signal in the direction of the rotating axis of the photosensitive drum (a primary scanning direction). In a state where the photosensitive drum is rotated, the light beam is scanned to carry out an exposure, thereby forming a latent image. If the rotation of the photosensitive drum is unstable and a vibration or an unevenness is caused, picture quality is deteriorated due to so-called banding phenomenon in which a stripe appears in a secondary scanning direction.
A factor for causing the rotation of the photosensitive drum to be unstable and generating the vibration or unevenness includes the structure of the gear train of a driving system, the member abutting against the outer periphery of the photosensitive drum such as the cleaner, a rotary developing unit for carrying out a developing color switching operation in the case of full color image forming apparatus.
The cleaner serves to clean toner remaining on the photosensitive drum after the transfer when the toner image formed on the photosensitive drum is transferred onto the transfer member. In order to enhance the cleaning effect, a proper member such as a blade member is used, and furthermore, is caused to abut on the outer peripheral surface of the photosensitive drum at a constant pressure by a spring, thereby removing the residual toner from the photosensitive drum.
If the abutment pressure is low, it is a matter of course that the cleaning effect is reduced. If the abutment pressure is raised, an irregular vibration is generated with frictional sliding between the outer peripheral surface of the photosensitive drum and the cleaner. Moreover, an abutment angle of the blade member with respect to the outer peripheral surface of the photosensitive drum, or the position of a supporting point for supporting the cleaner relative to the tangential line of the photosensitive drum at the abutment point also give an influence, as explained below.
For example, since the cleaner slidably comes in contact with the surface of the photosensitive drum, a vibration is applied to the rotation of the photosensitive drum unless the cleaner is caused to slide smoothly. When a rotating velocity fluctuation such as the vibration is generated during a period for which the surface of the photosensitive drum is exposed, an electrostatic latent image cannot be formed with high precision, resulting in a deterioration in the quality of an image.
As compared with an image forming apparatus in which a developing roller always abuts on a photosensitive drum, it is observed that the velocity fluctuation of the photosensitive drum is apt to be caused in an image forming apparatus in which a developing roller is retracted from the developing position of the photosensitive drum or carries out a development switching operation. For example, in a color image forming apparatus comprising a rotary developing unit which is rotated and moved by a developing color switching operation and the mounted developing cartridge repetitively comes in contact with or separates from the photosensitive drum at each time. Accordingly, the state of the outer peripheral surface of the photosensitive drum is not held uniformly between the contact state and the separating state, the frictional sliding operation of the cleaner cannot be carried out smoothly and the speed of the photosensitive drum thus becomes unstable.
More specifically, for example, in an image forming apparatus wherein images of respective colors for two sheets are formed and developed during one circulation of an intermediate transfer belt as shown in FIG. 3, the driving operations of various motors for the photosensitive drum and the intermediate transfer belt are started and an image signal is then turned ON based on image data, so that the surface of the photosensitive drum is exposed to write an electrostatic latent image. In a case where the image forming apparatus comprises a rotary developing unit, it is necessary to carry out a color switching operation for effecting a developer (developing roller) corresponding to the color of image data, prior to the exposing process. Thereafter, the developing operation is started together with the exposure of the surface of the photosensitive drum.
The cleaner slides on the outer periphery of the photosensitive drum on which residual toner of the previous image forming operation has been removed and cleaned, until a position “m” at which the developing roller is first brought into contact reaches the cleaner with toner. Since the toner also serves as a lubricant agent between the cleaner and the photosensitive drum, the vibrations or the rotation velocity fluctuation of the photosensitive drum tends to be occurred during the above period. This period corresponds to a period indicated by t′ in FIG. 3. Further, during a period “t” shown in FIGS. 2 and 3, relatively large velocity fluctuations can be observed. In such a condition, a position shift is caused on an electrostatic latent image so that the quality of an image to be formed is deteriorated. In FIG. 3, “n” indicates a position on the photosensitive drum from which the developing roller is separated, and “o” indicates a position on the photosensitive drum which is first subjected to the exposure.
The above explanation is for the case where a full color image is formed. However, the same phenomenon can be observed in a case where a monochrome color image is formed with a single color of toner as shown in FIG. 35. The same characters have the same meanings as those in FIG. 3 and repetitive explanations for those will be omitted. Further, the same phenomenon can be observed also in an image forming apparatus dedicated to form a monochrome image.
In the above image forming apparatus, the developing roller is rotated while being abutted against the outer periphery of the photosensitive drum, thereby transferring toner to the surface of the photosensitive drum to develop an electrostatic latent image as a visible toner image through electrostatic adsorption. Therefore, the toner is uselessly rubs and is thus deteriorated, and furthermore, the developing roller itself is also abraded slightly and deteriorated. In a case where an image having a single color, that is, a monochrome image is to be formed, particularly, the developing roller does not need to be switched and the developing roller is uselessly rotated for a long time as shown in FIG. 35. Particularly, a deterioration in the developer for storing black toner is promoted more quickly than that in other developers.
To suppress this deterioration, the rotation of the developing roller may be stopped (i.e., separated from an area on the photoconductive drum in which no latent image is formed) as shown in FIG. 36. However, in such a configuration, the rotation velocity fluctuation of the photosensitive drum may be generated not only when a latent image for the first page is formed but also a latent image for the third page is formed. In FIG. 36, the same characters have the same meanings as those in FIG. 3 and repetitive explanations for those will be omitted.